In general, RFID systems consist of a tag or multiplicity of tags implemented to provide information such as identity, features, or characteristics of an object to which the tag is affixed, and to transmit that information via an RF signal to a RFID reader in response to an RF interrogation signal received by the tag from the reader. In most instances of supply chain tagging applications the tag is placed on a container (e.g., a carton, a case or a pallet) for a multiplicity of the same items, in contrast to item-level RFID tagging, in which each individual item is given its own RFID tag. The identity of and perhaps other information relating to the tagged article is stored in a memory device of its tag, and is transmitted by the RFID tag to a remote interrogator, or reader, in response to a scan (or query, command or interrogation—these terms, for present purposes, meaning the same thing) from the reader when the reader is within the response range of the tag, i.e., a range suitable for RF communication between reader and tag. Thus, although the term RFID has a connotation of one-way transfer of identification information from an object (the tag) to another location, RFID systems often involve two-way communication.
In its most basic form, the conventional RFID tag consists of a transponder and an antenna. Often, the RFID tag itself is referred to as a transponder. These tags are in use in a variety of applications beyond supply-chain tagging, such as tracking movable assets (e.g., as diverse as rail cars and locomotives to cattle and other animals), non-stop highway toll collection, control of access to everything from secure areas of a facility to entertainment events, vehicle registration, mobile electronic payment of services, and a host of other applications where moderate communication distances and moderate data transfer are required, notwithstanding potentially difficult environments and high speed of tagged objects. These applications require that the RFID tag be small in size.
RFID tags may be either passive or active. A passive RFID tag lacks an internal self-sufficient power supply, e.g., a battery, and relies instead on the incoming RF query by the reader to produce sufficient power in the tag's internal circuitry to enable the tag to transmit a response. In essence, the query induces a small electrical current in the tag's antenna circuitry, which serves as the power source that enables tag operation. A passive tag may have range and function more limited than an active tag.
But the absence of a battery leads to certain advantages, primarily that a passive tag can have virtually unlimited life and be fabricated at much less cost and in considerably smaller size than an active tag, thus serving an important need to improve the efficiency and accuracy of tracking systems for commerce, security and defense. With costs of production trending downward, passive RFID tags could soon replace the ubiquitous universal product code (UPC) for many applications, the bar code strip found on myriad products and product containers in the stream of commerce. Unlike RFID, the imprinted bar code strip requires a line of sight optical scan to produce the UPC readout and the resulting computerized display or printout of price (at a point of sale of the bar-coded product) and other information regarding the product.
The on-board battery of an active RFID tag can give the tag a greater dynamic range than a passive tag, higher data rates and additional functions that require a constant supply of power, but the active tag has the aforementioned disadvantages of limited life and higher cost and size relative to the passive tag. The battery itself may be quite small, but not small enough to overcome the size disadvantage.
The principles of the present invention are applicable to both passive and active RFID tags, but have relatively greater impact for passive tags.
RFID tags may operate as read-only (RO) devices, capable of transmitting only fixed, invariable information stored in the tag memory of the semiconductor integrated circuit (IC) chip in which the tag is fabricated, as the readout when the tag is scanned by the reader in an RF communication between reader and tag. RFID tags may also be readable/writable devices adapted to allow their memories to be read and/or overwritten by a reader during a communication session. Data stored in memory (e.g., electrically-erasable programmable read-only memory, or EEPROM), whether original, overwritten or new, is available for transmittal to the reader on receipt by the read/write RFID tag of an appropriate command. Tag memory may contain a RO portion and a read/write portion.
The form of communication known as modulated backscatter typically used by passive tags is a decades-old technique. Tags that communicate in this way can be very low power, with operational distances as great as tens of meters for radio signals in the ultra high frequency (UHF) or microwave bands.
Passive backscatter tags, or simply passive tags, typically use one or more Schottky diodes to convert the reader's RF signal incident on the tag's antenna into a rectified DC voltage. Often, such diodes are used in a voltage-doubler configuration to boost voltage. However, the impedance of the tag's electronics and Schottky diode power circuit are poorly matched to antenna designs heretofore proposed for RFID tags. Conventional matching techniques have been utilized in an effort to reduce the mismatch as much as possible to tolerable levels (e.g., as described in publications such as Application Note 1008, “Designing the Virtual Battery”, Hewlett-Packard Corporation, 1997 (the applicable divested technology group now under the Agilent banner); or U.S. Pat. No. 4,816,839 but they create disadvantages of RFID tag size, cost, capability and efficiency.
Passive RFID tags usually incorporate very simple antenna structures, principally dipoles, loops or patches, in linear or circular polarized designs with impedance matching elements. Typically, the antenna is embedded in or attached to the structure of the tag, and the antenna port has moderate impedance typically on the order of 20 to 300 ohms. In contrast, the impedance of the tag electronics is capacitive, with a typical impedance of 5−j350 ohms.
Multi-turn loop antennas can be used at low RF frequencies (in a range of about 100 kHz (kilohertz) to 13.56 MHz (megahertz)), but resonant-type antennas have enjoyed more common use in the UHF bands in the prior art. The latter include the fan-paddle of the aforementioned '839 patent, dipoles, folded dipoles, dipoles with parasitic elements, arrays of folded dipoles, loops, spirals and patches. The size of a resonant antenna is on the order of a half wavelength or more of the RF frequency at which it is operated. The physical size of antennas generally has been shortened by use of meander lines of various kinds. Antennas such as short dipoles have been used with matching techniques to compensate for the typically large mismatch between the antenna impedance and the load impedance.
Over the past several years, semiconductor technology has progressed to the point at which microwave Schottky diodes can, by use of CMOS (complementary metal oxide silicon) process technology, be integrated into the IC chip along with the other component circuitry of the tag's electronics. Thus, a RFID tag operating at UHF frequencies can be constructed as a single IC chip (i.e., as a radio frequency IC, or RFIC, or application-specific IC, or ASIC) together with an antenna on the same substrate. Such tags have been previously available for operation at low RF frequencies, typically at or below 13.56 MHz.
More recently, other options have been made possible for tag antennas through advances in process technology that have produced further reduction in chip size including the size of RFICs and ASICs. But antennas and impedance matching techniques heretofore proposed for passive RFID tags remain burdened by limitations on size, efficiency, cost, bandwidth, and sensitivity to nearby objects such as the surface on which the tag is mounted.
A typical conventional RFID tag reader employs a transceiver, a control unit and an antenna for communicating with (e.g., interrogating) the tag at one or more designated RF frequencies among several allocated for this purpose. U.S. Federal Communications Commission (FCC) radio regulations specify frequency and power permissible for RFID reader operation in the United States. Regulatory agencies in other countries have their own restrictions for radio communications. Consequently, the efficiency of a passive backscatter RFID tag in converting RF signals incident on the antenna from the reader to DC power for the tag's backscatter response, is of considerable importance to the operation and use of such devices.
In the specific case of a RFID system consisting of a reader (interrogator) and a tag (transponder) used to complete transactions in high speed applications such as toll collection transactions by identification of authorized vehicles passing the reader in designated lane(s) at highway speed, the system configuration poses serious engineering challenges. The RFID tag should be thin, small, of straight forward design and therefore relatively easy to manufacture, low cost, and high performance, capable of use in potentially hostile (or at least unfriendly) operating environments where the tag may be subject to extremes of vibration, chemicals, dust, temperature and humidity and other atmospheric or ambient conditions. While the utility and use of the present invention is explained in terms of an electronic toll collection, the present invention is also of great use in all other applications of RFID as well as use in systems containing fixed and handheld readers.
The design of RFID tags requires matching the antenna impedance and load impedance, usually by a matching circuit, for maximizing the RF power from the reader's interrogation or command signal received at the tag antenna to be delivered to the RFIC with minimum loss, and thereby achieve optimum tag sensitivity. The custom integrated circuit of which the RFIC is comprised may include the voltage-doubler, analog and digital circuitry of the transponder, and memory capacity to store the software programming and data to be transmitted to the reader in response to a command, as well as other electronics as may be necessary for a particular RFID design.
Theoretically, maximum power delivery is achieved by conjugate impedance matching, which demands that the impedance from the antenna be, as closely as possible, the mathematical conjugate of the RFIC input impedance. This represents an ideal impedance match.
The typical RFIC input impedance is generally a complex impedance of Z=5−j350 ohms, which can be normalized to Z=0.1−j6.9 ohms (where j is the imaginary portion of the complex impedance Z) for a 50-ohm system typical of the antenna port, as shown in the RFIC input impedance diagram (Smith chart) of FIG. 1.
For maximum power transfer through the conjugate impedance match, the impedance from the antenna should be close to Z=5+j350 ohms, which can be normalized to Z=0.1+j6.9 ohms for the 50-ohm system, as shown in the Smith chart of FIG. 2, representing the impedance from the antenna.
FIG. 3 is a Smith chart of the dipole antenna impedance in a typical prior art RFID tag. The antenna impedance, for example, is Z=18−j41 ohms at 915 MHz. And as illustrated by the chart of FIG. 4, a dipole antenna with the Smith chart of FIG. 3 exhibits a relatively constant gain of about 1.8 dB (decibels) from 0 through 360 degrees.
A significant problem encountered in seeking to use prior art RFID tags in high speed applications, such as highway on-the-fly (vehicle non-stop) toll collection systems is the degree of difficulty encountered to design an antenna and impedance matching circuit of reasonably practical size, to optimize the RF communications performance of the tag.
A resonant antenna, such as the dipole antenna that has been the antenna of choice for RFID tags, has an optimum size at about a half-wavelength of the designated RF frequency for communications between RFID reader and tag. For example, if the designated RF frequency is 915 MHz, which is typical for RFID communications, one-half wavelength is about 164 mm (millimeters). The impedance matching circuit of a dipole antenna, as well as other antenna designs used in RFID tags of previous design, has a relatively small shunt inductive impedance and large series inductive impedance. These attributes create a prodigious, virtually impossible task to design an impedance matching circuit of practical size and high energy efficiency for a dipole antenna or other antenna design heretofore proposed for use in a RFID tag.
Generally for antenna designs, the series-matching component may be easy to implement, while the shunt matching component is very difficult to implement because the circuit ground required for the shunt component is normally not well defined and not readily available without degrading antenna performance or necessitating an impractical antenna design.